In order to increase the capacity of a cellular radio communications network, different access strategies may be employed. Rather than assigning a particular frequency bandwidth to a single radio communication as in Frequency Division Multiple Access (FDMA)-based systems, time division multiplexing may be employed as in Time Division Multiple Access (TDMA)-based systems to increase the number of user communications that use the same frequency bandwidth. Multiple users transmit on the same frequency bandwidth, but at a different time. In Code Division Multiple Access (CDMA), multiple users share the same frequency bandwidth, but each radio communication is assigned a different spreading code used by a receiver to extract the desired information.
As third generation CDMA-based mobile radio communication systems like wideband CDMA and CDMA 2000 evolve, they may incorporate time division multiplex principles along with code division multiplexing. For example, a shared channel may be employed primarily for downlink (from base station-to-mobile station) packet data transmissions, i.e., a High Speed-Downlink Shared Channel (HS-DSCH). Such a high speed downlink shared channel corresponds to downlink spreading codes shared by mobile users on a time division multiplexed basis. For a particular time interval, the entire set of HS-DSCH spreading codes is used for downlink transmission to a single mobile terminal. In the example illustrated in FIG. 1, there are five spreading codes (SC) allocated to the HS-DSCH corresponding to SC1–SC5. The use of these five spreading codes is decided on a timeslot basis. Data to a first mobile terminal MT#1 is transmitted during timeslot TS1 employing all five spreading codes SC1–SC5. At timeslot TS2, all five spreading codes are employed to transmit data to the mobile terminal MT#2. At timeslot T3, all five spreading codes are used to transmit data to the mobile terminal MT#3. While this pattern may repeat itself, this need not be the case. For example, timeslots TS4 and TS6 could be used to send data to MT#1, and timeslot TS5 could be used to send data to MT#2.
In addition to sharing resources using time division multiplexing, high speed shared channel resources may also be shared by mobile users using Code Division Multiplexing (CDM). For a given time interval in CDM, data to multiple mobile terminals may be transmitted in parallel using different subsets of the spreading codes allocated to the high speed shared channel in addition to time division multiplexing. Referring to FIG. 2, during timeslot TS1, data to the mobile terminal MT#1 is transmitted using all five spreading codes. However, during timeslots TS2 and TS3, two spreading codes SC1–SC2 are used to transmit data to the mobile terminal MT#2, and spreading codes SC3–SC5 are used to send data to the mobile terminal MT#3. Similar to the pure TDM case, as described in the previous paragraph, this pattern may or may not repeat itself. For example, timeslots TS4 and TS6 could be used to allocate all five spreading codes to send data to MT#1, and timeslot TS5 could be used to send data to MT#1 using SC1–SC2 and data to MT#3 using SC3–SC5.
One issue regarding such a high speed downlink shared channel is how to offer satisfactory quality of service to all mobile terminals in the cell. Radio channel conditions vary dramatically and quite rapidly. It may be advantageous not to try to adjust the transmit power on the high speed downlink shared channel to compensate for quickly varying channel conditions. (It still may be desirable to adjust the transmit power for other reasons, e.g., to vary the fraction of the total downlink cell capacity allocated for high speed downlink shared channel transmission.) Instead of transmit power adjustment, the modulation and/or coding scheme used on the high speed downlink shared channel may be varied to adapt the high speed downlink shared channel transmission to varying channel conditions. This is referred to as adaptive modulation and coding (AMC). By varying the modulation and/or coding scheme, the high speed downlink shared channel data rate may be varied. For mobile terminals experiencing favorable conditions, e.g., the mobile terminal is close to the base station, higher order modulation, e.g., 16 QAM, and higher rate coding, e.g., R=¾, may be used. Similarly, for mobile terminals experiencing less favorable positions, e.g., the mobile terminal is close to the cell border, lower order modulation, e.g., QPSK, and lower rate coding, e.g., R=¼, might be used. Thus, mobile terminals experiencing favorable positions can be offered higher data rates, i.e., higher quality of service (QoS), while mobile terminals experiencing less favorable positions can be offered lower data rates, i.e., lower quality of service.
Each mobile terminal that may receive downlink packet data on the high speed downlink shared channel (HS-DSCH) may also communicate with the base station using a pair of associated uplink (UL) and downlink (DL) dedicated physical channels, i.e., UL DPCH and DL DPCH. The associated uplink and downlink dedicated physical channels correspond to uplink and downlink dedicated spreading codes. In contrast to an AMC-based approach for the high speed shared channel, “fast” power control is typically employed in existing CDMA systems to control the transmit power of CDMA signals. Fast power control should also be used to control the transmit power on dedicated channels, including downlink dedicated channels associated with the high speed downlink shared channel, in order to compensate for quickly varying channel conditions. That fast power control is usually implemented gradually using small, incremental (+/−) power control commands. The power control command that controls the transmit power of the downlink dedicated channel is carried on the uplink dedicated channel and vice versa.
In addition to carrying power control commands for downlink dedicated channels, an uplink dedicated channel may be used for uplink control signaling related to the high speed downlink shared channel, e.g., estimates of the downlink channel quality. Such HS-DSCH quality estimates may be used, for example, by the base station to select the high speed downlink shared channel modulation and/or coding scheme. The uplink dedicated channel may also carry other types of services such as speech. Similarly, the downlink dedicated channel may carry other services such as speech as well as downlink signaling information related to the high speed downlink shared channel. For example, such control signaling may indicate that data for a specific mobile terminal is being transmitted on the high speed shared channel along with information about certain transmission parameters such as a modulation and/or coding scheme to be used on the high speed shared channel. FIG. 3 illustrates an approach where each of four mobile terminals (MT1–MT4) is allocated its own dedicated uplink and downlink signaling channels. However, all mobile terminals may use a single downlink shared data channel, where sharing may be accomplished using time division multiplexing and/or code division multiplexing as described above. FIG. 4 illustrates these shared and dedicated channels with more specific labels. Instead of transmitting all downlink signaling related to the high speed shared channel on an associated downlink dedicated channel, some of the downlink signaling may be transmitted on an associated shared signalling channel (not illustrated in FIGS. 3 and 4).
CDMA-based systems are typically deployed with a one-to-one correspondence between uplink and downlink carrier frequencies (fDL,1⇄fUL,1; fDL2⇄fUL,2, etc.). However, it is likely that, in the future, more spectrum will be allocated for the downlink compared to that allocated for the uplink because there will be larger volumes of downlink traffic than uplink traffic, e.g., high speed multimedia services like downloading web page information to a mobile terminal. FIG. 5 illustrates frequency spectrum as trapezoids with multiple downlink carriers (fDL,1; fDL,2) sharing a single uplink carrier (fUL). Different downlink carriers may support different types of services. As illustrated in FIG. 6, one downlink carrier may carry only speech (fDL,1), and one downlink carrier may only carry packet data (fDL,2). Both downlink carriers could share the same uplink carrier (fUL) to carry a variety of uplink information including uplink speech, uplink packet data, uplink control signaling associated with the high speed downlink transmission, etc. One reason to separate different types of services on different carriers is that different services may have very different characteristics. As an example, a speech service may be more sensitive to interference as compared to a packet data service. Thus, a packet-data carrier can be loaded with more traffic compared to the case when the carrier is also carrying speech services.
Frequency reuse is a defining characteristic of cellular systems. In frequency reuse, the same carrier frequencies are used in multiple, geographically different areas for which the system provides coverage. Significantly, these areas are separated from one another by a sufficient distance so that any co-channel or adjacent channel interference is less than a particular threshold. FIG. 7A shows a cellular system with a frequency reuse of one, i.e., the same carrier frequency f1 is used in all cells. This is the case in CDMA-based cellular systems like CDMA 2000 and wideband CDMA. A frequency reuse of one means that the entire available frequency band is available in each cell. The entire available frequency band is represented symbolically in FIG. 7A as f1. However, that frequency band could be divided into, e.g., three subbands f1, f2, and f3, and in that case, every cell transmits over all three subbands.
The problem with a frequency reuse of one is the high level of inter-cell interference, i.e., interference originating from neighbor cells. To reduce inter-cell interference, FDMA and TDMA cellular systems typically use a frequency reuse greater than one, which means that neighboring cells use different carrier frequencies. FIG. 7B shows an example frequency reuse equal to three. Both of the examples of FIG. 7A and FIG. 7B are somewhat simplified in the sense that the uplink and the downlink typically use different carrier frequencies. Thus, f1 may be interpreted as a pair of frequencies [fUL,1, fDL,i].
For a CDMA system that employs a high speed downlink shared channel, a frequency reuse of one leads to large variations in the channel quality, (measured, for example, in terms of signal-to-interference ratio (SIR)), between different positions in a cell. Larger variations in channel quality may result from higher levels of downlink interference that may be present in the cell, especially close to the cell border. As described above, using adaptive modulation and/or coding, the data rate of the high speed downlink shared channel depends on the channel quality, e.g., the detected SIR. Thus, with a frequency reuse equal to one, there may be large variations in the high speed downlink shared channel services offered to different mobile terminals, depending on their position in the cell. Mobile terminals close to the cell site and far from the cell border may experience high downlink signal-to-interference ratios, allowing for high data rates on the HS-DSCH. Mobile terminals close to the cell border may well experience lower, downlink signal-to-interference, allowing only low data rates on the downlink channel. A reuse of greater than one results in less inter-cell interference at the cell border, improves channel quality, and accordingly, allows for significantly higher data rates at the cell border over the high speed downlink shared channel.
Although there is a benefit in terms of lower inter-cell interference, and thus improved services for mobiles at cell borders achieved by employing a frequency reuse greater than one for a CDMA-based system, there are certain disadvantages with using a frequency reuse greater than one in a CDMA-based cellular system. If the total available uplink spectrum is less than the total available downlink spectrum, as illustrated in the example of FIG. 6 above, there may not be enough uplink spectrum to support a frequency reuse greater than one on the uplink. One solution to this problem is to use a frequency reuse equal to one for the uplink, i.e., uplink communication is performed on the same carrier frequency fUL in all cells. At the same time, a frequency reuse greater than one is used for the downlink, i.e., downlink communication is performed on different carriers in neighbor cells. However, this solution causes a problem related to soft handover.
Soft or diversity handover is readily supported in CDMA systems where the frequency reuse is one. Soft handover is typically used in a CDMA system with an uplink a frequency reuse of one, in order to avoid excessive uplink interference and significant capacity loss. In soft handover, an uplink transmission from a mobile station is received by multiple neighbor base stations, e.g., base stations in a so-called “active set.” In addition, all base stations in the active set transmit on the downlink to the mobile station. Each of the power control commands transmitted from all base stations in the active set is considered by the mobile station when it regulates its uplink transmit power. Mobile uplink transmissions are usually simultaneously power controlled from all base stations in the active set in such a way that if any of the base stations requests a reduction in power, the mobile terminal transmit power is reduced. The mobile terminal transmit power is only increased if all base stations in the active set request an increase of the transmit power. If a frequency reuse equal to one is used on the downlink, the mobile terminal only has to receive a single carrier frequency to receive the power control command signals from all of the base stations in the active set.
If there are multiple carrier frequencies in a system with a frequency reuse greater than one for the downlink, the task of receiving power control commands from the active set of base stations is more complicated. The mobile terminal must be able to receive and process simultaneously the same signals on different frequency carriers. Because this is complicated and requires a multi-carrier receiver in the mobile terminals, in practice, soft handover is easier to implement when the downlink frequency reuse is one. In addition, the ability to perform soft handover associated with a frequency reuse of one is beneficial for some services like speech in both uplink an downlink directions. Soft handover permits seamless handover between base stations, leading to improved quality of service.
The present invention resolves these competing interests with respect to frequency reuse in a CDMA-based mobile communication system (although it is not limited to CDMA systems). Different frequency reuse values are associated with different channels in a cellular communications system, e.g., different types of channels. For a high speed downlink shared type of channel, the frequency reuse may be greater than one in order to achieve higher data rates. On the other hand, the frequency reuse may be set to one for other channel types, e.g., dedicated channels including both uplink and downlink dedicated channels. Alternatively, the frequency reuse may also be set greater than one for channels in addition to a downlink shared channel, e.g., one or more dedicated downlink channels, while a frequency reuse of one is deployed for one or more uplink dedicated channels. Using different frequency reuse values reduces inter-cell interference, particularly at cell borders, while still maintaining existing soft handover schemes for dedicated channels if one or more of the channels is a CDMA channel.